1. Field of the Invention
This invention relates to optical devices and in particular to display devices.
2. Art Background
Liquid crystal materials are presently employed in a wide variety of display devices. Various expedients are utilized to produce a desired optical change upon application of an electric field. The applied field induces a change in the liquid crystal. The particular change induced depends on the phase of the liquid crystal being employed. The phsyical configuration of the device, e.g., the placement of polarizers, is designed to yield an observable optical change in conjunction with the specific spatial change induced in the molecules of the liquid crystal bulk.
Liquid crystals exist in a number of phases and the ambient temperature strongly influences the phase in which a liquid crystal is found. (See G. W. Gray and P. A. Winsor, Liquid Crystals and Plastic Crystals, Vols. 1 and 2, Halsted Press (1974); G. R. Luckhurst and G. W. Gray, The Molecular Physics of Liquid Crystals, Chapters 1 and 12, Academic Press (1979); and D. Demus et al, Flussige Kristalle in Tabellen, VEB Verlag, Leipzig, (1974) for a description of these phases, specific liquid crystals which exhibit these phases, and typical ranges of temperatures for transitions between these phases.) Thus, since a device is designed for a particular phase of the liquid crystal being utilized, the device is useful only in the temperature range where the liquid crystal exists in the desired phase.
In the most commonly employed device configuration, i.e., a twist cell, a liquid crystal in its nematic phase is utilized. (See G. W. Gray, Advances in Liquid Crystals, BDH Publications (1979) for a description of twist cells.) Although devices based on nematic liquid crystals have achieved wide spread use, they typically do not have memory, i.e., upon application of an electric field the liquid crystal changes state and in the particular device configuration being employed produces an optical change. However, upon removal of this field, it reverts to its original physical and optical state.
Memory is a desirable property for devices employed in large scale displays which are matrix addressed. In such displays the matrix is typically formed from an array of liquid crystal device elements. Electrodes (x and y electrodes) are employed to apply an electric field to a given matrix element. The total field applied to an element is the resultant field produced by the voltage applied through the x and through the y electrode. The x and y electrodes are also arranged to form a matrix with a liquid crystal display element at each intersection of a x and a y electrode. By applying a suitable voltage to the appropriate x and y electrode, the optical state of the matrix element at the intersection of these electrodes is changed. Thus, by using n number of x electrodes and m number of y electrodes, mxn elements are controlled. If the matrix elements are bistable--if they require an electric field to change from a first state to a second and the application of another field to change back--the voltages applied to the x and the y electrodes need not be sustained to maintain the optical change which they induce. As a result of this attribute, a considerable simplification in the electronics necessary to control a display array is achieved.
A variety of methods have been proposed to produce a bistable device--a device that upon application of a field assumes a second state which persists after removal or diminution of the field until a second field is applied. One of these methods depends upon the use of liquid crystals having a smectic C, I, or F state. (See N. A. Clark and S. T. Lagerwall, Applied Physics Letters, 36(11), 899 (1980).) (The smectic I or F liquid crystals are not as fluid, and are not preferred since they switch relatively slowly.) Basically, this device utilizes the two spatial orientations of the polarizability vector in a ferroelectric, smectic C phase liquid crystal. These two orientations typically are stable and are separated by a potential energy barrier. Once one state is induced, it requires energy to convert the liquid crystal from one spatial orientation state within the smectic C phase to the other. Thus, a bistability is inherent in the material. To employ this bistability, a device is fabricated by contacting the liquid crystal material with two electrodes in a manner which produces a succession of smectic C phase layers, 5, as shown in the FIGURE. (The liquid crystal material is initially brought into this layered structure by placing a liquid crystal mass in the smectic A phase between the two electrode plates, bringing the plates close together (for example, within 10 .mu.m or less) and sliding the plates relative to each other.) A change of state in a given region of the liquid crystal material is produced by applying an appropriate field to the electrodes bounding this region. (The field need not be maintained.) A second field is applied in turn, to return the liquid crystal material to its initial state. An optical change accompanying the change in state is produced by, for example, orienting crossed polarizers above and below the electrode-liquid crystal-electrode sandwich, with the direction of one polarizer aligned with the direction of the long axis of the liquid crystal molecules in either one of the two spatial orientations.
The sole liquid crystal materials suggested for use in a smectic C bistable device are decyloxybenzylidene p'-amino-2-methylbutylcinnamate hexyloxybenzylidene p'-amino-2-chloropropylcinnamate. Although these materials are useful, they have a variety of shortcomings. The most significant of these problems is that these materials exist in the smectic C phase only in a very narrow temperature range and that the materials are not particularly stable. Thus, bistable devices depending on these liquid crystal materials are somewhat limited shelf life and are not useful in applications which require device operation in a wide range of temperature conditions. Additionally, these liquid crystal materials have a smectic C phase only well above room temperature. Obviously, a device employing these materials would not be suitable for room temperature applications and thus, is of diminished utility for many applications.